Francesca --
If you did the calculations yourself (i.e. you have the firm
characteristics for all t, and you are calculating the measure of
growth), you may want to rethink your specification. Can you take the
log of #employees at each t, and model that as a function of the RHS
variables using panel methods? Or use #employees as your dependent
variable, and use -xtpoisson-. Either method produces coefficients
that can be interpreted as "effects" in terms of growth (e.g. a one
unit change in RHS variable X is estimated to produce a .05 unit
change in lnY, which corresponds a five percent increase over the
prior year).
On 9/6/06, Francesca Gagliardi <francesca.gagliardi@unical.it> wrote:

Dear Nick and Al,
Thank you very much for your replies to my question. I apologise for not
having specified better how my dependent variable has been obtained. It is
just a growth rate of firms, calculated as (employees at time t - employees
at time t-1)/employees at time t-1. I have 15678 observations, of which
23.7% are negative values, 39.2% are zeros and the remaining are positive
values
Thanks again for your interest.